Adjuvant therapy with different bacillus Calmette-Guérin (BCG) preparations is a well-established guideline-endorsed treatment for nonmuscle invasive bladder cancer (NMIBC). Our observational study demonstrates equality between BCG and mitomycin C (MMC) treatment based on the oncological outcome. However, there were significant toxicity differences with higher rates in the BCG treatment group. The potential adverse effects of BCG in terms of a BCGitis are controversially discussed regarding their occurrence. As such, we sought to retrospectively evaluate the incidence in 106 consecutive patients. The BCG group demonstrated minor adverse effects in 78.4% and major adverse effects in 43.3%-partially coincident. Moreover, the parallel MMC group showed in 34.7% respectively 1.4% adverse events-as expected distinctly lower. In the context of this clinical discussion, we refer to alternative treatment concepts. Our data show a high clinical relevance of the patient's primary comorbidity.